I guy I know at a local dirt work contractor is kind of a jack of all trades for them. He drives the lowboy, dump truck, and turns wrenches. Will run backhoe or mini ex in a pinch. I think he likes doing a little bit of everything. Some guys just want to stay in a excavator and make a big hole. They don't want to jump from one thing to another.
I don't think there will be a huge difference in the money, whether you are operating, driving or turning wrenches. In fact if you are full time over the road driving, I would say you will probably make less money staying home and working local. If you are a good truck driver, most employers would rather pay you to do that well, as to pay you to learn to run equipment, which you are not going to be as good at starting out. It's going to come a little at a time. Such as when the loader man doesn't show up and you can help out. Or they are just short a skid loader man for a afternoon, cleaning up a jobsite.
I would say the turning wrenches would be the most physical amount of work, then operating, then driving truck.